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Arsenic in drinking water is known to pose a health risk to humans, including the risk of cancer. Now a new study has revealed that the carcinogenic effects of arsenic exposure via drinking water may lie dormant for years before cancer develops.

For the study, which was recently published in the Journal of the National Cancer Institute, researchers tracked mortality rates of Chilean people living in a region where they were exposed to drinking water contaminated with arsenic. The study presents evidence of increased bladder, kidney and lung cancer as much as forty years after exposure to high levels of arsenic ceased.

Inorganic arsenic is a naturally occurring drinking water contaminant that is found in high concentrations in drinking water in many countries around the world, posing a severe public health threat to millions globally. Health issues associated with extended exposure to arsenic in drinking water include bladder, kidney and lung cancer as well as skin cancer, cardiovascular disease, and other negative health effects.

In 1958, the drinking water supply for the northern Chilean city of Antofagasta experienced a dramatic rise in arsenic concentrations. In 1970 the local water treatment plant upgraded their facility to include a mechanism for removing arsenic from the drinking water, which led to a dramatic drop in arsenic exposure. As Antofagsta is recognized as the driest place on the planet inhabited by humans, everyone residing there at the time had no other option but to drink the water supply provided by the city, even though it was know to contain high levels of arsenic.

The study examines the health and mortality of residents exposed to arsenic over time, and identifies 'a clear relationship between and cancer mortality rates'. The researchers found that mortality rates from bladder, kidney and lung cancer started to rise about ten years after residents where exposed to high levels of arsenic, only peaking more than twenty years after measures were put in place to reduce residents exposure to arsenic, with associated cancer mortality rates remaining high for as much as 40 years after residents were exposed to the highest levels of arsenic.

While the researchers hope to continue monitoring this population, from the results of the study to date, they have already concluded that the delay between arsenic exposure and the onset of arsenic-related cancers makes it a human carcinogen with one of the longest dormancy periods. These findings are not only important in terms of the light they shed on latency patterns of carcinogenic contaminants, they may also have a direct implication on public health.

The lengthy latency period following exposure to arsenic means that arsenic-related health effects may only materialize years later, which in effect means that the incidence of these types of cancers are likely to remain high amongst populations exposed to high level of arsenic long after these exposures have ended.

However, it may be possible to take proactive measures to improve the health outcomes of people who have been exposed to high levels of arsenic in the past.

According to the authors, "possible long-term interventions to reduce mortality and morbidity after high exposures end include disease screening, reducing important co-exposures, heavy metal detox treatments, health services resource planning, and increasing public awareness of arsenic health effects."